1. Field of the Invention
This invention relates to a method of forming a polarization inversion region by applying an electric field across a ferroelectric substance crystal. This invention particularly relates to a polarization inversion method for a ferroelectric substance, wherein an electrode having a predetermined pattern is formed on a surface of a ferroelectric substance crystal, and an electric field is applied across the ferroelectric substance crystal in this state in order to cause polarization inversion to occur in accordance with the shape of the electrode. This invention also relates to a process for producing an optical wavelength converting device, wherein the polarization inversion method for a ferroelectric substance is utilized.
2. Description of the Related Art
A technique for performing wavelength conversion of a fundamental wave to its second harmonic by use of an optical wavelength converting device provided with regions, in which spontaneous polarization (domain) of a ferroelectric substance having a nonlinear optical effect has been inverted periodically, has been proposed by Bleombergen, et al. (The proposed technique for performing the wavelength conversion is described in Phys. Rev., Vol. 127, No. 6, 1918, 1962.) With the proposed technique for performing the wave length conversion, phase matching between the fundamental wave and its second harmonic is capable of being effected by setting a period Λ of the polarization inversion regions at integral multiples of a coherence length Λc, which is represented by Formula (1) shown below.Λc=2π/{β(2ω)−2β(ω)}  (1)wherein β (2ω) represents the propagation constant of the second harmonic, and β (ω) represents the propagation constant of the fundamental wave.
In cases where the wavelength conversion is performed by use of a bulk crystal of a nonlinear optical material, the wavelength at which the phase matching is effected is limited to a specific wavelength inherent to the crystal. However, with the proposed technique for performing the wavelength conversion described above, in cases where the period Λ of the polarization inversion regions, which satisfies Formula (1) shown above, is selected with respect to an arbitrary wavelength, the phase matching (i.e., the so-called “pseudo-phase matching”) is capable of being effected efficiently.
One of techniques for forming the periodic polarization inversion structure described above has been disclosed in, for example, U.S. Pat. No. 5,594,746. With the disclosed technique for forming the periodic polarization inversion structure, periodic electrodes having a predetermined pattern are formed on one surface of a single-polarized ferroelectric substance crystal having a nonlinear optical effect, corona charging of the ferroelectric substance crystal is performed with the periodic electrodes and a corona wire, which is located on the side of the other surface of the ferroelectric substance crystal opposite to the one surface described above, in order to apply an electric field across the ferroelectric substance crystal, and local area limited polarization inversion is thereby caused to occur at regions of the ferroelectric substance crystal, which regions stand facing the periodic electrodes.
Besides the technique for forming the periodic polarization inversion structure by the utilization of the corona charging, a technique for forming the periodic polarization inversion structure by the utilization of an electron beam has been proposed in, for example, U.S. Pat. No. 5,249,250 and a literature “FABRICATION OF DOMAIN REVERSED GRATINGS FOR SHG IN LiNbO3 BY ELECTRON BEAM BOMBARDMENT,” ELECTRONIC LETTERS, Vol. 26, No. 3, pp. 188–189, February 1990. With the proposed technique for forming the periodic polarization inversion structure by the utilization of an electron beam, an electron beam is irradiated onto one surface of a single-polarized ferroelectric substance crystal having a nonlinear optical effect, an electric field is thus applied across the ferroelectric substance crystal, and a local area limited polarization inversion region is thereby formed in the ferroelectric substance crystal.
In cases where the electric field is applied across the ferroelectric substance crystal by use of the periodic electrodes described above, it is necessary that the side of the periodic electrodes is grounded. The periodic electrodes are constituted of a plurality of electrodes which are arrayed with a predetermined period. Therefore, in order for the periodic electrodes to be grounded, ordinarily, a technique for applying the electric field across the ferroelectric substance crystal is employed, wherein a single connecting electrode, which is electrically connected to the periodic electrodes, is formed on the surface of the ferroelectric substance crystal and is electrically connected to the ground via a wire.
In cases where the aforesaid technique for applying the electric field across the ferroelectric substance crystal is employed, if the connecting electrode is located within, for example, the regions, which are to be corona-charged, or the regions, to which the electron beam is to be irradiated, the polarization inversion will also occur in the areas of the ferroelectric substance crystal, which areas stand facing the connecting electrode. In such cases, if the amount of inverted electric charges is large, depending upon the thickness of the ferroelectric substance crystal, the area of inversion, and the period, it will often occur that the areas of the ferroelectric substance crystal, which areas stand facing the connecting electrode, suffer from breakage (cracking, and the like). Particularly, the crystal breakage occurs primarily at a bent area of the connecting electrode, which bent area is electrically connected to the periodic electrodes described above.
Also, particularly, the aforesaid problems with regard to the crystal breakage are markedly encountered with an LiNbxTa1−xO3 crystal, where 0≦x≦1, or an LiNbxTa1−xO3 crystal, where 0≦x≦1, having been doped with MgO, ZnO, or Sc, which crystal has the characteristics such that an electric conductivity of the ferroelectric substance crystal changes largely before the polarization inversion occurs and after the polarization inversion has occurred, and the electric charges are apt to concentrate at initial inversion areas.
Besides the cases where the periodic polarization inversion regions are formed by use of the periodic electrodes described above, the aforesaid problems with regard to the crystal breakage are also encountered in cases where the electric field is applied across the ferroelectric substance crystal with corona charging or electron beam irradiation by use of the electrode having a predetermined pattern and the connecting electrode electrically connected to the electrode having the predetermined pattern.
If the crystal breakage occurs, the ferroelectric substance crystal will become a defective produce, and the yield of polarization inversion will become low. Also, in cases where the optical wavelength converting device described above is produced by forming the periodic polarization inversion structure in the ferroelectric substance crystal with the aforesaid technique for applying the electric field across the ferroelectric substance crystal, if the yield of polarization inversion is low, the cost of the optical wavelength converting device cannot be kept low.